Fourth generation and beyond
Nakano did not write about anything past the third generation of robots. But we might mention a 4th-generation robot: a machine of a sort which is yet to be deployed. An instance is a fleet or population of robots that reproduce and evolve. Past that, we may say that a 5th-generation robot is something humans have not yet dreamed of or, if someone has thought up to such a thing, he or she has not published brainstorm.